Subphylum Chelicerata

The subphylum Chelicerata, made up of chelicerates, includes spiders, scorpions, mites, sea spiders, and horseshoe crabs. Chelicerates lack antennae and typically have six pairs of appendages. The first pair of appendages, the chelicerae, are modified into pincers or fangs. The major group in Chelicerata is class Arachnida (uh-RAK-ni-duh), which has more than 70,000 species.

Class Arachnida

Members of the class Arachnida, called arachnids, include spiders, scorpions, mites, and ticks. Like crayfish, arachnids have a body that is divided into a cephalothorax and an abdomen. The cephalothorax usually bears six pairs of jointed appendages: one pair of chelicerae; one pair of pedipalps, which aid in holding food and chewing; and four pairs of walking legs.

Anatomy of a Spider

Spiders range in length from less than 0.5 mm long to as long as 9 cm (3.5 in.) long in some tropical tarantula species. As Figure 36-9 shows, the body of a spider is very narrow between the cephalothorax and the abdomen.

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